The so-called heat-curing type resinous compositions have been widely used as molding, impregnation, lamination and injection molding materials as well as various electric insulation material, structural plastics, coating material, adhesives and the like. Recently, with an increasing demand of using such materials under much severe conditions, heat-resistance of such materials become the center of public concerns.
Since the plastics formed by the reaction of polyisocyanate and polyepoxide have excellent thermal properties, many efforts are concentrated in the development works of this novel type of resinous materials.
It is in general recognized that the resins obtained by the reaction of polyisocyanates and polyepoxides are excellent in heat-resisting properties, but are undesirably hard and brittle and have the problems of deficient impact strength.
Excellent heat-resisting properties are believed to be due to their specific ring structure having oxazolidone group and isocyanurate group produced by the reaction of the indicated materials, and poor impact strength and the like are to the presence of excessively highly crosslinked structures derived from trimerized isocyanurate groups. Therefore, it is expected that stiff resins might be obtained by the reaction of polyepoxide and polyisocyanate if only something be devised to give polyoxazolidone containing the least amounts of trimerized isocyanurate.
Oxazolidone ring-formation reaction is in general carried out without using a catalyst at an elevated temperature. When adopted, the following catalysts are generally used.
That is, tertiary amines (e.g. imidazole, hexamethylene tetramine), tetraethyl ammonium iodide, combination of aluminium chloride and pyrrolidone, combination of aluminium chloride and phosphoric ester, lithium halide, complex of lithium halide and phosphoric acid (e.g. complex of lithium bromide and tributyl phosphine oxide) and the like. Recently, Matuda et al reported in J. Org. Chem. p.2177 to 2184, (12) 1986 and Chemistry Letters, p.1963 to 1966, 1986, complexe of organo tin halide and Lewis base or stibonium as a novel oxazolidonation catalyst. Marks, Morris and Brebees Leimond disclosed in Japanese Patent Publication (unexamined) 500730/87 organoantimony compounds as an effective catalyst for the preparation of polyoxazolidone resin.
In the case of reaction between mono-functional compounds, i.e. monoepoxide and monoisocyanate, the desired oxazolidone compound may be obtained in the presence of such catalyst, though the reaction must be carried out at a higher temperature for a long period of time. However, in the case of reaction between highly functional polyepoxide and polyisocyanate, the reactions involved are more complicated and a quantity of undesired products are always by-produced. Such by-products are essentially trimerized isocyanurate and polyether produced by the homopolymerization of epoxide. Among them, trimerized isocyanurate may give highly crosslinked, brittle polymer and therefore, trimerization reaction must be effectively controlled. Nevertheless, heretofore proposed catalysts for the formation of polyoxazolidone, are not only effective for the oxazolidone formation reaction, but also for the isocyanurate formation reaction and therefore, the polyoxazolidone products prepared by the known processes do always include 30 mol % or more isocyanurate. Furthermore, in the reaction between isocyanate and epoxide, with the increase in the amounts of said epoxide, a strong exotermic reaction is liable to occur, causing scorch in the formed moldings and heat deterioration of the molded article For this reason, it has long been desired to provide a method for the preparation of polyoxazolidone containing the least amounts of trimerized isocyanurate by the reaction of polyepoxide and polyisocyanate under mild conditions.
It is, therefore, an object of the invention to provide a heat-curing type resinous composition capable of resulting a high quality plastic of polyoxazolidone through the reaction of polyepoxide and polyisocyanate components and to provide a molded article with excellent heat-resisting properties composed of polyoxazolidone containing the least amount of trimerized isocyanurate.